Monitoring integrity of structures—such as bridges, pipelines, machinery, and railways—requires monitoring various structural quantities at critical points. Of special interest, is to monitor strain, and acoustic emission. Acoustic emission waves are high frequency stress waves that are generated in materials at time of creation of growth of cracks.
Monitoring strain is a reflection of loading condition in a structure, and it is known that cracks are created in materials is highly correlated with strain, and often, excessive strain leads to creation and growth of cracks. Creation and growth of cracks causes propagation of acoustic emission waves in materials. Therefore, monitoring acoustic emission events is important to detect creation and growth of cracks.
Conventional methods for attaching foil strain gauges to objects are complicated and often, attachment is not easy especially when attachment is done in field (e.g., at a highway bridge site). A conventional system and method for monitoring strain in an object will now be described with reference to FIGS. 1A-H.
FIGS. 1A-H illustrate steps needed to attach a conventional strain gauge to an object 100.
FIGS. 1A-B show the top and the side views of object 100, respectively.
FIGS. 1C-D show top view and side view of object 100 after paint is removed from an area. To prepare for attachment of a conventional strain gauge, first, surface of object 100 is cleaned. If the surface of object 100 painted, paint is removed from an area 102 of object 100 to which the conventional strain gauge is intended to be attached.
FIGS. 1E-F show the top view and the side view of object 100 after a conventional strain sensing element 104 is attached thereto.
Strain sensing element 104 is attached to object 100 at exposed spot 102 using an adhesive. For strong bonding, often high quality adhesives are used for attaching strain sensing element 104 to the surface of object 100. In this example, exposed spot 102 is shown as having a rectangular shape. This non-limiting example shape and size is used merely for purposes of discussion.
FIGS. 1G-H show top view the side views of object 100 after electronic signal conditioner 108 is connected to strain sensing element 104. In this example, the conventional strain gauge includes a foil sensing element 104, wire leads 106 and an electronic signal conditioner 108. Electronic signal conditioner 108 includes an amplifier (not shown), a power supply (not shown) and control modules for measuring strain. Foil sensing element 104 is operable to detect strain in object 100 and generate a strain signal based on the detected strain. Wire leads 106 provide the strain signal from fail sensing element 104 to electronic signal conditioner 108 for processing.
A difficulty of conventional system and method for attaching a strain gauge to an object, as discussed above, is that often, the foil strain gauge element is extremely delicate and working with it requires extreme care. For example, approximate thickness of commercially available foil strain gauge elements is typically 50 micrometers. This thickness shows the degree of delicacy and sensitivity of strain sensing elements, and extreme care that should be practiced during the attachment in order to avoid permanent damage to the strain sensing element. While practicing the above attachment procedure may not be difficult in a laboratory environment, it is very difficult task in field, where attachment must be done at the place of a structure. An example of attachment of strain gauge in field is accurate strain measurement on massive structures or object such as highway bridges to monitor its structural integrity or loading conditions. Working with extremely delicate and sensitive foil strain gauges is a very difficult job in the field.
What is needed is a device and method that enables strain and acoustic emission monitoring for long times in situations that energy supply available to the device is limited. Example situations are when the device is battery operated, or when the energy consumption of the device is supplied through an ambient source such as a photovoltaic cell.